1. Field
Apparatuses and methods consistent with the exemplary embodiments relate to a fusing unit and an image forming apparatus employing the same, and more particularly, to a fusing unit and an image forming apparatus employing the same which prevents loss of an image and a contamination of a print medium due to a static electricity during a fusing operation.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus forms an electrostatic latent image by scanning light to an image carrier charged by a predetermined electric potential, develops such image with a predetermined toner, transfers and fuses such image on a paper and thereby prints an image. The electrophotographic image forming apparatus includes a transfer unit which transfers an image developed on the image carrier to a print medium and a fusing unit which fuses the image transferred to the print medium by heat and pressure.
FIG. 1 illustrates major parts of a conventional electrophotographic image forming apparatus.
As shown therein, the conventional electrophotographic image forming apparatus includes a transfer roller 3 which transfers a toner image T developed on an image carrier 1 to a print medium M, and a fusing unit 10 which fuses the toner image T transferred to the print medium M.
The fusing unit 10 includes a fusing belt 13 which has a heating lamp 11 mounted therein, a nip plate 15 which supports the fusing belt 13 to form a fusing nip N, and a pressure roller 17 which faces the nip plate 15, interposing the fusing belt 13 therebetween. The fusing belt 13 includes a non-conductive surface contacting the print medium M, and is charged by an electric potential having the same polarity as the toner image T on the print medium M and prevents an image from being transferred to the fusing belt 13 during a fusing operation. The fusing unit 10 heats and presses the non-fusing toner image T transferred to the print image to thereby fuse the toner image T on the print medium M when the print medium M passes the fusing nip N.
With the foregoing configuration, the image forming apparatus charges the print medium M with a high voltage by applying up to 3 kV voltage to a transfer roller 3 during a transfer process depending on the environmental conditions and the type of a print medium. Accordingly, the toner image T of the image carrier 1 is transferred to the print medium M in a transfer nip. If the print medium M is charged as above, a gap discharge can be generated from a front area of the transfer nip (area A in FIG. 1) and an image defect may occur. In consideration of the foregoing issue, the conventional image forming apparatus includes a discharging plate 5 which has an end part shaped like a saw tooth and applies a voltage to a print medium M before the print medium enters into the fusing nip N, and applies a voltage opposite in polarity to a charging voltage to the print medium M through the transfer roller 3 immediately after the transfer operation.
Applying a voltage through a discharging plate 5 lowers an adherence of the non-fused toner image T to the print medium M and causes blur or scattering, or the toner forming the image may be detached from the print medium M and scatter. The scattered toner may contaminate internal and external sides of the image forming apparatus and the print medium.
Also, static electricity is generated by friction between the fusing belt 13 forming the fusing nip N during the fusing process and the pressure roller 17 and the print medium M. If the generated static electricity is not controlled normally, a loss of an image may arise due to the static electricity from the fusing nip N. As a measure for preventing the loss of the image, a surface of the fusing belt 13 or the pressure roller 17 contacting the print medium M includes a conductive material, and is connected to the ground through a predetermined electrode in the conventional art. Otherwise, a voltage is applied through a power source 19 connected to the pressure roller 17 as shown in FIG. 1.
In the case of applying the voltage, a positive bias voltage of +100 to 1,000V is applied to the pressure roller 17 through the power source 19 so that the toner image T charged with a negative electric charge maintains its adherence to the print medium M. Then, the adherence of the toner image T to the print medium M increases, but the surface of the fusing belt 13 and the pressure roller 17 is charged with electricity by the applied voltage and easily contaminated by the toner or paper dust.
Particularly, immediately after the print medium M passes the fusing nip N, static electricity accumulated in the print medium M contacts a predetermined location P1 of the fusing belt 13 as shown in FIG. 2 and dozens of or thousands of electric potentials are discharged. The static electricity discharged as above remains in the surface of the fusing belt 13 including a non-conductive material and have properties different from other surfaces of the fusing belt 13. That is, the surface of the fusing belt 13 has a negative electric charge while the predetermined location P1 has a positive electric charge. Accordingly, when the fusing belt 13 rotates during the fusing process, the predetermined location P1 repeatedly faces the print medium as shown in FIG. 3. FIG. 3 illustrates a type {circle around (1)} and a type {circle around (2)} which represent a variation to a surface electric potential of the fusing belt 13 and a variation to the surface electric potential of the pressure roller 17, respectively, when the fusing belt 13 consecutively rotates. Referring to the surface electric potential of the fusing belt 13, it can be known that the maximum electric potential in the location P1 is +516V.
Accordingly, when the location P1 of the fusing belt 13 is located in the front side of the fusing nip N, the toner image charged with the negative electric charge is separated from the print medium M and attached to the fusing belt 13, and thus image separation and a white band or a black band may arise in another location of the print medium on a regular basis due to the separated image. Also, the separated image is attached to the pressure roller 17 and may contaminate a rear surface of the print medium M or another part of the print medium M.
A resistance of the print medium is changed depending on its type, thickness and environmental conditions. For example, in the case of a print medium which has a relatively high resistance property in a low-temperature and low-moisture environment, a voltage is applied to the pressure roller 17 corresponding to the resistance to overcome such resistance, and the problems due to the application of the voltage may worsen.